Methods for preparing polymer beads of uniform particle size (UPS) by suspension polymerization are generally carried out by jetting an organic phase of polymerizing monomers containing initiators in an immiscible liquid phase containing suspension stabilizers through capillary openings to form monomer droplets in the liquid phase and then polymerizing the monomer droplets to polymer beads. The polymer beads are generally converted into various ion exchange resins. Cation exchange resins are produced by sulphonation of the polymer beads, whereas chloromethylation and amination of the polymer beads give anion exchange resins. In case of acrylic cation exchange resins subsequent hydrolysis and aminolysis of acrylic anion exchange resin is carried out after acrylic polymerisation. Ion exchange resins have a variety of applications, for example, they are used in various water treatment applications, deacidification or dealkalisation applications, removal of contaminants or impurities, decolouration of sugar or water or recovery of metal.
Koestler and Robin describe a method for preparing polymer beads of uniform particles size by suspension polymerization comprising passing a monomer liquid into a first column (droplet formation column) from one end of the first column through a perforated disc having orifices, cocurrently passing a monomer immiscible aqueous liquid from the said one end of the first column, passing the droplet-containing liquid from the other end of the first column into the upper section of a second column (gelation column) and cocurrently introducing into the upper section of the second column, a downflowing aqueous stream to partially polymerise the droplets, allowing the partially polymerized droplets dispersed in the aqueous liquid to flow from the lower section of the second column into a separation vessel for gravity separation of the droplets and for concentration of the droplets in the aqueous liquid and allowing a comparatively concentrated slurry of the droplets to flow from the separation vessel to a bank of agitated polymerization reactors, wherein the droplets are further polymerized to polymer beads (U.S. Pat. No. 3,922,255).
The above method requires a large number of equipments for the formation and concentration of the droplets, namely a droplet formation column, a gelation column for partial polymerization and stabilisation of the droplets and a separation vessel for concentration of the droplets and formation of a droplet slurry. Because of the large number of equipments, the cost of the equipment configuration for carrying out the method is very high. Cycle time for carrying out the method is also increased as the monomer and aqueous solutions and the droplets pass through a series of equipments in a complicated and zig-zag manner. Further, the monomer phase and aqueous phase enter the droplet formation column from the bottom and side thereof, respectively and in the gelation column, the liquid containing droplets enter from the side thereof, whereas the aqueous stream enters from the top thereof. As a result, there are chances for obstruction of flow and coalescence of the droplets in the droplet formation column and gelation column thereby affecting the uniformity of the droplets and ultimately that of the polymer beads.
Lange and Striiver describe a process for the production of polymer beads of improved uniform particle size and uniform quality which comprises feeding a monomer solution and an aqueous solution immiscible with the monomer solution cocurrently into a reaction column from the bottom thereof using concentric dual nozzles. The column comprises a droplet formation section, encapsulation section and shell hardening section. Droplets of the monomer are formed in the aqueous solution in the droplet formation section of the reaction column. The droplets are encapsulated and hardened in the encapsulation and shell hardening sections of the reaction column respectively by feeding encapsulating and shell hardening components into the respective sections of the column from the side thereof. The temperatures in the various sections of the reaction column are different and are independently controlled with monitoring devices. The encapsulated and shell hardened droplets are allowed to flow down from the reaction column into a polymerization vessel. Because of the encapsulation, the droplets will be stable during agitation in the polymerization vessel, wherein the droplets are further polymerized to polymer beads. Following polymerization, the encapsulation is removed (U.S. Pat. No. 4,427,794).
In the above process, the monomer droplets are formed, encapsulated and shell hardened in three different sections of the same reaction column and the different temperatures in the various sections of the reaction column are independently controlled with monitoring devices. Besides, it involves removal of the shells of the droplets. As a result of all this, the cycle time increases and the process is expensive and difficult and cumbersome to carry out. The temperature monitoring devices increase the capital and maintenance cost of the reaction column and reduces reliability. Due to the temperature gradient across the various sections of the reaction column, the droplets may experience thermal shocks. There are also chances for obstruction of flow and coalescence of the droplets due to the flow of the encapsulating and shell hardening components from the side of the reaction column. Thermal shocks and coalescence of the droplets will reduce the uniformity of the polymer beads.
Timm teaches a methods and an apparatus for preparing uniform particle size polymer beads. A monomer jet having laminar flow characteristics is formed by flowing a monomer phase comprising a polymerizable monomer through an orifice plate having a plurality of orifices into a continuous suspension phase comprising a liquid immiscible with the monomer and a stabilizer. The monomer jet is vibratorily excited to breath it into droplets comprising the monomer. The droplets are polymerized into spheroidal polymer beads in a polymerisation vessel (U.S. Pat. No. 4,444,961).
Vibratory excitation is given to the monomer jet by vibratory devices such as mechanical, electroacoustic, hydroacoustic or electromagnetic vibrators or magnetoresistive transducers all of which increase the cost in terms of capital investment, maintenance and energy consumption and also reduce reliability. Due to the vibratory excitation, there is also possibility for the droplets to coalesce thereby reducing the uniformity of the particle size distribution of the polymer beads. Further, the liquid phase enters the apparatus from the side thereof, whereas the jetting of the monomer phase through the orifice plate at the bottom of the apparatus enters the liquid phase in the upward direction. As a result of this also, there are chances for obstruction of flow and coalescence of the droplets thereby further reducing the uniformity of the particle size distribution of the polymer beads.
Miyata et al describe a method for preparing an oil-in-water type uniform dispersion of liquid droplet. The method comprises ejecting an oil type monomer into an aqueous medium containing a dispersion stabilizer, which forms a continuous phase and moves upwards, a hydrophobic liquid having a specific gravity smaller than the aqueous medium, through a nozzle plate having a plurality of perforations capable of ejecting the hydrophobic liquid upwards, to form liquid droplets of the hydrophobic liquid in the aqueous medium. The nozzle plate comprises a number of perforations for ejection of the hydrophobic liquid arranged in a ring form around the center portion where no perforation is provided, ie without uniformly providing such perforations over the entire surface whereby it is possible to efficiently form an excellent dispersion without swaying of ejected hydrophobic liquid streams in the vicinity of perforations for ejection and provide a large amount of perforations for ejection as a whole and improve the productivity of the dispersion.
In case the method is concerned with the preparation of polymer beads of uniform particle size, the hydrophobic liquid is a polymerizable monomer containing a polymerization initiator. A dispersion of a polymerizable monomer of uniform droplet size is formed in an apparatus having an outlet at an upper portion, and inlets for a monomer and an aqueous medium at a lower portion. The dispersion is discharged continuously from the upper outlet of the apparatus and introduced into a polymerization reactor for polymerization into polymer beads of uniform size. An aqueous medium containing a dispersion stabilizer is continuously supplied to the apparatus from the side thereof for forming the dispersion of the monomer of uniform droplet size, which is filled with the aqueous medium, to form a continuous phase of the aqueous medium moving upwards and being continuously discharged from the upper outlet of the apparatus. The forward end of the monomer inlet at the lower portion of the apparatus is provided with nozzle plate for forming monomer droplets with the aqueous phase.
In Miyata et al, the monomer solution and aqueous solution are fed counter currently and there are chances for the monomer droplets to coalesce and obstruct upward movement of the droplets in the aqueous solution thereby reducing the uniformity of the particle size distribution of the polymer beads. Further a greater force is required to be exerted on the jetting to facilitate upward movement of the monomer droplets in the aqueous solution thereby increasing the power requirement of the apparatus.
EP 2088161 A1 describes a method for preparing monodisperse crosslinked bead polymers comprising introducing droplets having a harmonic mean size from 50 to 1500 microns and comprising at least one monomer, at least one crosslinker and a free-radical polymerization initiator into an aqueous medium through orifices in a droplet forming vessel to produce an aqueous suspension of unencapsulated droplets having a volume fraction of droplets from 35 to 64%. The aqueous suspension of droplets is allowed to flow in a downward direction in a pipe such that ratio of droplet harmonic mean size to inside pipe diameter is from 0.001 to 0.35, mean linear flow velocity is from 0.5 to 2.5 ft/s (0.15 to 0.75 m/s) and temperature is maintained at least 20° C. below a temperature at which the polymerization initiator has a half-life of 1 hour. The monomer droplets are polymerized in a polymerization reactor set up at lower level with respect to the droplet forming vessel.
The monomer droplets are formed by known methods including vibrational jetting and natural jetting. Flow control of polymer droplets under gravity into the polymerization reactor is quite difficult and there are chances for the droplets to coalesce and obstruct flow of droplets thereby affecting the uniformity of the particle size distribution of the polymer beads. Further vibrational jetting mechanisms will increase the cost in terms of capital investment, maintenance and power consumption and reduce reliability.
There is thus still need for methods and equipments for preparing polymer beads of uniform particle size by suspension polymerization, which are simple and easy to carry out and which are cost effective and which give polymer beads of improved uniform particle size and quality in a reduced cycle time.